1. Field of Invention
The present invention relates to a glass substrate-holding tool. The glass substrate-holding tool according to the present invention is useful for holding a glass substrate during the production of a reflective mask blank for EUV (Extreme Ultra Violet) lithography (hereinbelow, referred to as “EUV mask blank” in the specification) to be used for, e.g. the production of semiconductors, and during the production of a functional film-provided substrate for such an EUV mask blank.
Further, the present invention relates to a method for producing an EUV mask blank and a functional film-provided substrate for such an EUV mask blank by employing the glass substrate-holding tool according to the present invention.
2. Discussion of Background
In the semiconductor industry, a photolithography method using visible light or ultraviolet light has been employed as a technique for transferring a fine pattern on a silicon substrate or the like, which is required for forming an integrated circuit including such a fine pattern. However, the conventional photolithography method has reached near to the limit while semiconductor devices have had finer patterns at an accelerated pace. In the case of the photolithography method, it is said that the resolution limit of a pattern is about ½ of an exposure wavelength, and that even if an immersion method is employed, the resolution limit is about ¼ of an exposure wavelength. Even if an immersion method using an ArF laser (193 nm) is employed, it is estimated that the resolution limit is about 45 nm. From this point of view, EUV lithography, which is an exposure technique using EUV light having a shorter wavelength than ArF lasers, has been considered as being promising as the exposure technique for 45 nm or below. In this specification, it should be noted that the phrase “EUV light” means a ray having a wavelength in a soft X ray region or a vacuum ultraviolet ray region, specifically a ray having a wavelength of about 10 to 20 nm, in particular of about 13.5 nm±0.3 nm.
It is impossible to employ a conventional dioptric system like photolithography using visible light or ultraviolet light since EUV light is apt to be absorbed by any substances and since the refractive index of the substances is close to 1 at this wavelength. For this reason, a catoptric system, i.e., a combination of a reflective photomask and a mirror, is employed in EUV light lithography.
A mask blank is a stacked member to be used for fabrication of a photomask, which has not been patterned yet. In the case of an EUV mask blank, it has a structure wherein a substrate made of glass or the like has a reflective layer for reflecting EUV light and an absorber layer for absorbing EUV light, formed thereon in this order. As the reflective layer, a Mo/Si multilayer reflective film is usually employed wherein molybdenum (Mo) layers as high refractive layers and silicon (Si) layers as low refractive layers are alternately stacked to increase a light reflectance when irradiating a layer surface with EUV light.
As the absorber layer, a material having a high absorption coefficient to EUV light, specifically, e.g. a material containing chromium (Cr) or tantalum (Ta) as the main component, is employed.
The multilayer reflective film and the absorber layer are formed on an optical surface of a glass substrate by, e.g. an ion beam sputtering method or a magnetron sputtering method. At the time of forming the multilayer reflective film and the absorber layer, the glass substrate is held by a holding tool. Although a mechanical chuck and an electrostatic chuck are used as a glass substrate-holding tool, it is preferred in terms of a reduction in dust generation that such an electrostatic chuck be used as the glass substrate-holding tool to catch and hold the glass substrate at the time of forming the multilayer reflective film and the absorber layer, in particular at the time of forming the multilayer reflective film.
The electrostatic chuck is a technique which has been heretofore used to catch and hold a silicon wafer in a process for producing semiconductor devices. The electrostatic chuck holds a silicon wafer by an electrostatic attractive force which is generated by bringing a central portion of the silicon wafer into contact with a catching and holding surface having a planar shape in a circular shape, a rectangular shape or the like, and applying a voltage across electrode portions of the electrostatic chuck.
For catching and holding a glass substrate, the electrostatic chuck is also used such that a central portion of the glass substrate, specifically a central portion of a rear surface of the glass substrate opposite to the film deposition surface of the glass substrate with a multilayer reflective film and an absorber layer expected to be stacked thereon during the production of an EUV mask blank, is brought into contact with the catching and holding surface of the electrostatic chuck, for, e.g. reasons that the shape of the electrode portions is not complicated, that it is possible to provide a sufficient holding force, and that the caught and held glass substrate is prevented from inclining. It has been heretofore considered that it is preferred to catch and hold such a central portion of the glass substrate since it is possible to make device design simple and since it is possible to reduce costs by utilizing an electrostatic chuck widely used for catching and holding a silicon wafer.
Hereinbelow, the surface of a glass substrate with a multilayer reflective film or an absorber layer expected to be stacked thereon during the production of an EUV mask blank is referred to as the “film deposition surface” of the glass substrate, and the rear surface of the glass substrate opposite to the film deposition surface is referred to as the “rear surface” in the specification.
However, when a glass substrate is held by bringing a central portion of the rear surface of the glass substrate into contact with the catching and holding surface of an electrostatic chuck, it is likely that foreign substances deposit on the central portion of the rear surface of the glass substrate, or that the central portion of the rear surface of the glass substrate is scratched. In the case of a glass substrate used for production of EUV mask blank, the central portion of the rear surface of the glass substrate is a portion that is also usually designated as a quality-guaranteed region for every device, such as an exposure system, and is required to be free from the deposition of foreign substances or the occurrence of scratches. From this point of view, the deposition of foreign substrates or the occurrence of scratches on the central portion of the rear surface could cause a serious problem.
It appears to be sufficient to bring an outer edge portion of the rear surface of a glass substrate except for the quality-guaranteed region into contact with the catching and holding surface of an electrostatic chuck in order to prevent the deposition of foreign substances or the occurrence of scratches on a central portion of the rear surface.
However, when the glass substrate is held by bringing the outer edge portion of the rear surface into contact with the catching and holding surface of the electrostatic chuck, the surface area of a portion of the glass substrate in contact with the catching and holding surface of the electrostatic chuck (hereinbelow referred to as “the caught and held portion” of a glass substrate in the specification) is reduced. Accordingly, when an attempt is made to provide a catching and holding force enough to hold the glass substrate, the pressure per unit area applied to the caught and held portion is increased, which is likely to cause a problem of occurrence of scratches on the caught and held portion or of occurrence of foreign substances caused by such scratches. Further, a large amount of charged foreign substances are likely to be attracted by an electrostatic field generated at the caught and held portion.
Since the caught and held portion is present on the outer edge portion of the rear surface, the effect caused by the occurrence of scratches or foreign substances is minor than a case where a similar problem is caused on the quality-guaranteed region of the rear surface. However, it is likely that foreign substances formed at the caught and held portion or a portion of foreign substances attracted to the caught and held portion is transferred to the quality-guaranteed region of the rear surface. When the caught and held portion is scratched, it is likely that the force required for holding the glass substrate is reduced in a post-process for mask blank produced on the glass substrate. Specifically, an electrostatic chuck is used as the glass substrate-holding tool to catch and hold a glass substrate in a mask patterning process for fabricating reflective masks from an EUV mask blank, or in handling of reflective masks for exposure in EUV lithography. If the caught and held portion is scratched to form a step, it is likely that the flatness of the caught and held portion is degraded to reduce the catching and holding force of the electrostatic chuck.
On the other hand, when an attempt is made to reduce the catching and holding force of an electrostatic chuck in order to prevent the caught and held portion from being scratched or a large amount of foreign substances from being caught on the caught and held portion, the catching and holding force applied to a glass substrate become insufficient, which leads to a case where the glass substrate is likely to be displaced or come off during the production of EUV mask blank.
Although explanation has been made about a case where a glass substrate is caught and held by an electrostatic chuck, there is a tool for holding a glass substrate by a mechanical chuck, which physically holds the outer edge portion of a glass substrate.
Patent Documents 1 and 2 listed below disclose that the outer edge portion of a glass substrate is held by a mechanical clamping tool (Patent Document 1) or a glass substrate-pressing tool (Patent Document 2) during the production of mask blank.
When such a tool is used, the surface area of the contact portion of such a tool in contact with a glass substrate is small. Accordingly, when such a tool is set to provide a holding force enough to hold the glass substrate, the pressure per unit area applied to the caught and held portion of the glass substrate is increased, which leads to a case where it is likely to cause a problem of occurrence of scratches on the caught and held portion or of occurrence of foreign substances caused by such scratches as in the electrostatic chuck.